There is known in the art injection molding apparatus for use in forming compact discs including an upper movable mold member and a lower stationary mold member. When the mold members are moved together, they form a cavity into which the molten material is injected through all opening in the upper dye half. The lower dye half supports a sprue injector including a sleeve and a knockout pin. The sleeve is formed with an internal recess at its upper end into which the plastic material flows when the material is injected into the mold cavity. After the plastic material has been injected into the mold cavity, the assembly is cooled and the sprue injector is retracted to cut the center opening in the disc being molded. This cutout portion of the disc forms part of the sprue which must be ejected. As the ejector assembly is withdrawn, ultimately the knockout pin butts against a stop so that upon continued withdrawal of the ejector assembly, the knockout pin forces the sprue out of the lower mold member. This is the ordinary and desirable sequence of operations in molding a compact disc.
The normal cycle of the apparatus described above is under ten seconds. In the course of each cycle the temperature varies between in excess of 300 degrees Celsius and about 70 degrees Celsius. It sometimes occurs that in the course of what is designed to be the sprue ejection operation, the knockout pin drives through the sprue to eject the center portion thereof while leaving the surrounding portion on the sprue ejector. This probably occurs because the sprue has not yet cooled to a fully solid state. The sprue material remaining after such a malfunction hardens on the ejector and prevents injection of material during the next cycle. Thus, the apparatus shuts down. Before production can be resumed, the stuck sprue material must be removed. In the prior art, this material must be melted out of the tool by use of a torch. It will readily be appreciated that this is a tedious job which interrupts production for an inordinately long period of time.